Many wells for oil and gas production are drilled by using a string of drill pipe drilled to a selected depth. Then the operator retrieves the drill pipe and runs casing into the well bore, which is then cemented to line the well bore. In another technique, rather than using drill pipe, the operator uses the string of casing as the drill string. When reaching the proper depth, the operator cements the casing string in the well.
A joint of drill pipe is a steel pipe with an externally threaded end and an internally threaded end. Drill pipe is thick-walled and designed for being made up and broken out many times. A joint or section of casing is a pipe with a thinner wall and designed to be made up only a few times at most. Normally, both ends of the casing joint are externally threaded, and a casing collar is secured to one of the ends. The casing collar has internal threads for connecting to the external threads of the adjacent casing joint.
There are also two different ways to rotate a drill string, either by a kelly bushing or by a top drive. The kelly-bushing technique imparts rotation to the drill string by causing the powered rotary table to rotate a square, tubular kelly attached to the top of a string of drill pipe. This technique is used only when drilling with drill pipe. Another technique is to use a top drive, which can be used both for casing drilling or drill pipe drilling. The top drive includes a power source, such as a hydraulic or electrical motor, that imparts rotation to the drill string and moves up and down the derrick. The top drive has a quill that it rotates, and either the string of drill pipe or the string of casing is connected to the quill. For casing drilling, the operator attaches a casing gripper to the quill. The casing gripper has grapples that when actuated either grip the inside or the exterior of the string of casing to impart rotation and also support the weight of the casing. Top drives with casing grippers can also be employed to run casing in well bores that have been drilled with conventional drill pipe.
Whether running casing in a previously drilled well or drilling with casing, the operator of a top drive rig uses basically the same technique. The casing already deployed is suspended by slips at the rotary table. The operator picks up a new joint or section of casing pipe, typically with a set of elevators, which comprises a clamp-like device that is carried on bails attached to the top drive assembly. The operator places the elevators around the new joint of casing, then picks the casing joint up with the top drive. The operator stabs the lower end of the casing joint into the upper end of the casing string suspended by the slips. The operator lowers the casing gripper into the casing collar at the upper end of the casing joint to be attached, then actuates the gripper to grip the pipe. The operator then rotates the top drive to secure the upper joint of casing to that suspended in the well. The operator then lifts the entire string with the top drive and either begins drilling or lowers the entire string if running casing.
While running and drilling with casing have many advantages, there is a risk that the internal threads in the casing collar at the upper threaded end of the casing joint being lifted will be damaged by the casing gripper as the gripper enters the casing collar. The casing gripper is inserted while the casing collar is suspended above the rig floor the length of the joint, which may be 30 to 40 feet. Consequently it is difficult for the driller to guide the casing gripper into the casing collar.